Modelling the nesting-habitat of threatened vulture species in the caucasus: An ecosystem approach to formalising environmental factors in species distribution models

Abiotic factors play an important role in species localisation, but biotic and anthropogenic predictors must also be considered in distribution modelling for models to be biologically meaningful. In this study, we formalised the biotic predictors of nesting sites for four threatened Caucasian vultures by including species distribution models (wild ungulates, nesting tree species) as biotic layers in the vulture Maxent models. Maxent was applied in the R dismo package and the best set of the model parameters were defined in the R ENMeval package. Performance metrics were continuous Boyce index, Akaike's information criterion, the area under receiver operating curve and true skill statistics. We also calculated and evaluated the null models. Kernel density estimation method was applied to assess the overlap of vulture ecological niches in the environmental space. The accessibility of anthropogenic food resources was estimated using the Path Distance measure that considers elevation gradient. The availability of pine forests (Scots Pine) and wild ungulates (Alpine Chamois and Caucasian Goat) contributed the most (29.6% and 34.3%) to Cinereous Vulture (Aegypius monachus) nesting site model. Wild ungulate distribution also contributed significantly (about 46%) to the Bearded Vulture (Gypaetus barbatus) model. This scavenger nests in the highlands of the Caucasus at a minimum distance of 5–10 ​km from anthropogenic facilities. In contrast, livestock as a food source was most important in colony distribution of Griffon Vulture (Gyps fulvus). The contribution of distances to settlements and agricultural facilities to the model was 45%. The optimal distance from Egyptian Vulture (Neophron percnopterus) nesting sites to settlements was only 3–10 ​km, to livestock facilities no more than 15 ​km with the factor contribution of about 57%. Excluding the wild ungulate availability, the ecological niches of studied vultures overlapped significantly. Despite similar foraging and nesting requirements, Caucasian vultures are not pronounced nesting and trophic competitors due to the abundance of nesting sites, anthropogenic food sources and successful niche sharing.